Highest-Q 25pF-40pF variable capacitors for VHF?

[szoftveres]

Hi All,

What are the recommendations for high-Q variable capacitors in the 25pF-40pF range?
So far I've found the old style ceramic trimmer to work the best, but I'm curious if there are any other good options. Anything else I should try?

Thanks!

[radiolistener]

high-Q variable capacitors in the 25pF-40pF range?

КП1-4, 5-100pF, 25 kV vacuum variable capacitor:


it cost about 75 USD on internet.

Very good for magnetic loop antenna 

Ups... didn't notice you're needs it for VHF, КП1-4 max frequency is 30 MHz...

[David Hess]

Piston trimmers achieve Qs in the 1000s.

[TimFox]

Piston trimmers are great.
Most have one terminal to the mounting bushing (normally grounded) but some have two leads for non-grounded operation.
More expensive ones have sapphire dielectric between the internal moving piston and the outside cylindrical electrode;  cheaper ones use glass.  Others are PTFE or air dielectric.
If this be a onesie (not manufacturing) project, Surplus Sales of Nebraska has a good selection of pistons.   https://www.surplussales.com/Variables/PistonTrimmers/index.html
For example, on the "high-Q" page, their catalog no. CTP5452 is a Johanson 1.0 to 16 pF panel-mount unit, air dielectric, with Q > 3000 at 100 MHz, for $18.99.
On the "standard" page is CTP MPT62004A from JFD, Q not specified, 0.8 to 18 pF for $6.00
Both of these have enough change in C to reach your 25 to 40 pF range with a suitable fixed capacitor.  Higher capacitance units are available there.

[szoftveres]

Thanks all, the piston trimmer seems like a good choice. I'm building a coaxial type resonator filter for low power level on vhf.

[TimFox]

A piston trimmer with grounded bushing is also a good mechanical fit for a coaxial resonator.

[David Hess]

Thanks all, the piston trimmer seems like a good choice. I'm building a coaxial type resonator filter for low power level on vhf.

Oh really?  Many years ago I built a helical resonator.  The first version used plastic film trimmers, which was marginal, but the second used pistol trimmers and performed superbly.  Piston trimmers down below.  They have little screw on caps with a gasket so the external slotted screw for adjustment is not visible.

I made the coupling window too large, so mine is over-coupled, but that also means very low loss so it worked out well enough.